The advent of the laser has stimulated a great deal of interest in the prospect of transmitting information on an optical carrier. Two major advantages are gained by the use of an optical carrier; namely extremely high rate of information transmission and higher transmission efficiency. The first and second advantages result from the extremely high frequency of the optical carrier and the narrow beam of the energy whereby increased directivity occurs so that a substantially larger fraction of energy transmitted from a small aperture transmitter reaches a receiver than is generally possible with radio frequency or microwave carriers.
One technique which has been proposed for impressing digital information on a light beam carrier is the modulation of the light beam between two orthogonal polarizations in accordance with a serial digital data stream. For example, a digital one corresponds to one polarization state and a digital zero corresponds to a second polarization state which is orthogonal to the first. Examples of orthogonal polarization states are two linear perpendicular polarization states and two circular polarization states of opposite rotation sense.
In the transmission of such a polarization modulated light beam over an optical path which includes the atmosphere and perhaps other mediums, it is anticipated that the atmosphere, which may act as a non-linear and somewhat random transmission medium, and other non-linear mediums will distort the transmitted polarization states in a random manner putting some of the power of an intended transmitted polarization state, corresponding for example to a digital one, into the other polarization state, corresponding to an unintended digital zero. When power is received in both polarization states crosstalk is said to be present. If, for example, perpendicular linear polarization states, such as horizontal and vertical linear polarizations, are transmitted, nonlinear mediums can be expected to randomly distort these linear states to elliptically polarized states. Since elliptically polarized light has components of both horizontal and vertical polarized light, the difficulty encountered at a receiver in determining whether a digital one or a digital zero was transmitted should be apparent. This difficulty is made further acute when it is considered that various inherent noise components at the receiver also influence the decision process. As the difference between the powers in the intended and unintended polarization states becomes small and approaches the rms receiver noise there is a significant probability that some of the decisions will be made in error. Thus, polarization crosstalk seriously degrades the performance of a polarization modulated system.
Polarization crosstalk can also be introduced by optical components at the transmitter. For example, changes in the optical alignment or orientation of the transmitting laser and its associated modulator can cause crosstalk. In addition, the characteristics of the electro-optic modulator at the transmitter are temperature sensitive. Furthermore, rotating optical elements at the receiver may vary the polarization of the received optical signal causing crosstalk.
Heretofore, control techniques have been suggested in the prior art for stabilizing the output polarization of an electro-optic modulator in association with a laser. Two such prior art control techniques are disclosed in U.S. Pat. No. 3,284,632 to Niblack et al. and U.S. Pat. No. 3,532,891 to Simmons et al. These prior art control techniques are not, in general, adaptable for compensating for polarization biases introduced in the entire optical path including the atmosphere and the receiver, particularly in the transmission of arbitrarily fast data rates. Furthermore, these prior art control techniques are not independent of the state of the intended digital data.